Ecologic Study of Living Organisms 629 



(i) Currents of Water and Air: Water and air currents in any one 

 direction have a tendency to be a hindrance to locomotion of organisms, 

 even for those equipped to swim or fly. Continuous strong winds have 

 a tendency to move animals out of one area into another in the direc- 

 tion of the air currents. Strong water currents also have a similar eflfect. 

 Water currents have a tendency to carry nonmotile types in the direc- 

 tion of the flow of water. Only those animals particularly constructed 

 are able to counteract the water currents effectively. The "streamline" 

 construction of many aquatic forms is beneficial to them for locomotion 

 purposes and may be a great factor in their distribution. 



(j) Presence or Absence of Natural Barriers: All types of living or- 

 ganisms have certain kinds of environments which are conducive to their 

 dispersal. Any natural hindrances to dispersal are known as natural 

 barriers. Something which may be a barrier to dispersal for one species 

 may be a natural method of dispersal for another species. Water may 

 be a natural method of dispersal for fishes, but it may prove to be a 

 natural barrier for terrestrial forms unless the water is not too deep or 

 extensive. Mountains may be natural barriers for certain types of ani- 

 mals, even if they are normally terrestrial forms. In this case, altitude, 

 snow, ice, lack of proper vegetation for foods, shelter, home sites, etc., 

 may be influential factors. Plants of certain types, when absent from 

 certain regions, may serve as barriers to animal dispersal because those 

 animals depend upon such vegetation for foods, shelter, home sites, etc. 

 Earthquakes and volcanic activities may be barriers to the dispersal of 

 certain types of animals and plants. Floods may be barriers to certain 

 forms, while they may be used as methods of dispersal by others. 

 Whether a certain condition serves as a means of dispersal or as a bar- 

 rier depends upon the structural and physiologic properties of the par- 

 ticular organism in question. Heavy seeds, which cannot be easily car- 

 ried by animals or the wind, may have difficulty in passing over a moun- 

 tain or a large body of water. Lighter seeds may not be affected by 

 these same barriers. The swiftly moving parts of a stream may be a 

 barrier to the dispersal of the inhabitants of a quiet body of water be- 

 cause the environmental factors of the swift stream are different from 

 those of the quiet area. 



(k) Presence or Absence of Natural Methods of Dispersal ("High- 

 ways"): Most types of animals and plants have particular methods by 

 means of which they are dispersed. The seeds of the dandelion are so 

 constructed that they are easily carried by the wind. Sometimes when 



